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A330218 Least BII-number of a set-system with n distinct representatives obtainable by permuting the vertices. 6
0, 5, 12, 180, 35636, 13 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

A set-system is a finite set of finite nonempty sets of positive integers.

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793. We define the set-system with BII-number n to be obtained by taking the binary indices of each binary index of n. Every set-system has a different BII-number. For example, 18 has reversed binary expansion (0,1,0,0,1), and since the binary indices of 2 and 5 are {2} and {1,3} respectively, the BII-number of {{2},{1,3}} is 18. Elements of a set-system are sometimes called edges.

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

The sequence of set-systems together with their BII-numbers begins:

0: {}

5: {{1},{1,2}}

12: {{1,2},{3}}

180: {{1,2},{1,3},{2,3},{4}}

35636: {{1,2},{1,3},{2,3},{1,4},{2,4},{3,4},{5}}

13: {{1},{1,2},{3}}

MATHEMATICA

bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];

graprms[m_]:=Union[Table[Sort[Sort/@(m/.Apply[Rule, Table[{p[[i]], i}, {i, Length[p]}], {1}])], {p, Permutations[Union@@m]}]];

dv=Table[Length[graprms[bpe/@bpe[n]]], {n, 0, 1000}];

Table[Position[dv, i][[1, 1]]-1, {i, First[Split[Union[dv], #1+1==#2&]]}]

CROSSREFS

Positions of first appearances in A330231.

The MM-number version is A330230.

Achiral set-systems are counted by A083323.

BII-numbers of fully chiral set-systems are A330226.

Cf. A000120, A003238, A007716, A016031, A048793, A055621, A070939, A214577, A326031, A326702, A330098, A330101, A330195, A330217, A330229, A330233.

Sequence in context: A332466 A323565 A195538 * A047658 A290804 A353365

Adjacent sequences: A330215 A330216 A330217 * A330219 A330220 A330221

KEYWORD

nonn

AUTHOR

Gus Wiseman, Dec 09 2019

STATUS

approved

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Last modified December 9 23:05 EST 2022. Contains 358710 sequences. (Running on oeis4.)